Ether Cleavage with Organo‐Alkali‐Metal Compounds and Alkali Metals

Abstract Reactions with organo‐alkali‐metal compounds are mostly carried out in ether‐type solvents. It has long been known that ether cleavage takes place at the same time. The mechanism of the cleavage, in particular the fact that dialkyl ethers can be decomposed by a variety of mechanisms acting simultaneously, has only become clear in the most recent investigations. The observation that even with purely aliphatic ethers a considerable amount of Wittig ether rearrangement can occur is remarkable. Unusual secondary reactions can also occur by means of which alkyl‐ and aryllithium compounds in ether or tetrahydrofuran yield new organolithium compounds—sometimes with rearrangement. The reactions of alkyl aryl ethers with alkali metals are also varied and five different mechanisms for them have been discussed in the literature. It is interesting, for example, that the cleavage of anisole can be directed simply by changing the solvent so that in one case only cleavage of the aryl‐oxygen bond occurs and in the other almost 100% cleavage of the alkyl‐oxygen bond results. The formation of biphenyl as a by‐product upon cleavage of the alkyl‐oxygen bond in anisole was also puzzling. This was later shown to occur not through the dimerization of phenyl radicals but via 2‐methoxybiphenyl. The number of cleavage mechanisms reduces to two if one assumes two different σ * ‐radical anions as being intermediates in the kinetically controlled reaction. Comparison of the reactivity of thioethers and ethers reveals not only gradual but also fundamental differences—the cleavage of the thioethers is thermodynamically controlled.